§1. To promote the common pastoral action of different neighboring dioceses according to the circumstances of persons and places and to foster more suitably the relations of the diocesan bishops among themselves, neighboring particular churches are to be brought together into ecclesiastical provinces limited to a certain territory.
§2. As a rule, exempt dioceses are no longer to exist. Therefore, individual dioceses and other particular churches within the territory of some ecclesiastical province must be joined to this ecclesiastical province.
§3. It is only for the supreme authority of the Church to establish, suppress, or alter ecclesiastical provinces after having heard the bishops involved.
Source: Code of Canon Law (Vatican.va)